1. Field of the Invention
The invention relates to a heat transfer liquid being formed of molten potassium nitrate.
Products which are used as a heat transfer liquid, for example in the vulcanization of rubber, must comply with a number of specific properties, such as e.g.:
a) chemical statility in the chosen pressure and temperature range PA1 b) favorable material values (low viscosity, high thermal conductivity and specific heat) PA1 c) low vapor pressure PA1 d) low pour and solidification temperature PA1 e) non-toxic or non-caustic PA1 f) non-corrosive PA1 g) easy to dispose of if unusable.
2. Description of the Related Art
A known heat transfer liquid of a commercially available salt mixture is formed of a ternary mixture of 53 wt.% potassium nitrate, 40 wt.% sodium nitrite and 7 wt.% sodium nitrate and has a melting point of 142.degree. C. However, the sodium nitrite contained therein causes a number of disadvantages. Contact between the molten salt mixture and acid agents, for example carboxylic acids from organic material, leads to the formation of toxic nitrogen oxides, which require particular safety measures. Furthermore, if organic substances come together with sodium nitrite, deflagrations and explosions can occur. Moreover, a reaction of the nitrite with atmospheric oxygen takes place at elevated temperature to form nitrate, which manifests itself in a changed composition of the salt melt, associated with an increase in the melting temperature, and in the end results in the melt being unusable. Finally, because of the considerable toxicity, the disposal of spent nitrite-containing melts present problems and requires transportation to special dumps.
A ternary salt mixture of 44 wt.% calcium nitrate, 44 wt.% potassium nitrate and 12 wt.% sodium nitrate which has a relatively high melting point of 144.degree. C. and a high viscosity, which greatly impedes dripping of the melt from rubber profiles to be vulcanized, for example, is known from German Patent DE-PS 30 38 844.
A ternary salt mixture of 57.6 wt.% potassium nitrate, 16.2 wt.% sodium nitrate and 26.2 wt.% lithium nitrate which is said to have a melting point of 127.degree. C. is furthermore known from British Patent No. 992 418. In that case also, the addition of the sodium nitrate causes an increased viscosity of the melts, which can be a disadvantage in certain processes.